<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>css-chooser</title>
    <style>
        /* css基本选择器：id、class、标签 */
        /* id > class > 标签 */
        
        /* id选择器 */
        #id {
            background-color: red;  /*直接写颜色单词，比较low，适合临时演示用*/
        }
        /* class选择器 */
        .class {
            background-color: #ffff00;  /*十六进制写法*/
        }
        /* 标签选择器 */
        div {
           width: 100px;
           height: 100px; 

           background-color: rgb(0, 0, 255);    /*rgb写法*/
        }
    </style>
</head>

<body>
    <div id="id" class="class">id</div>
    <div class="class">class</div>
    <div>div</div>
</body>

</html>